La Nina brought flooding but climate change not off the hook

Neville Nicholls

'Global warming may be leading to a wetting trend across Australia.' Photo: AFP/Queensland Police Service

The past two years have been Australia's wettest two-year period since at least 1900. Not surprisingly, people ask whether global warming caused the record rains and floods. The simple answer is ''no'' - the heavy rains and floods have been caused by back-to-back La Nina events.

La Nina events and their ''dry'' brothers known as El Nino events are the extremes of a natural climate phenomenon called the El Nino Southern Oscillation, or ENSO, which has been producing Australia's ''droughts and flooding rains'' for thousands of years - since well before humans started pumping carbon dioxide into the atmosphere.

ENSO is caused by an interaction between the tropical Pacific Ocean and the atmosphere overlying it. Thanks to scientific research over the past century, we know quite a lot about how ENSO operates, including how to use it to predict year-to-year variations in Australian rainfall. Before both the last two summers, the Bureau of Meteorology predicted an increased chance of above-average rainfall, because the climate system was already in the middle of a strong La Nina event.

La Nina and El Nino events typically appear around March-April, increase in intensity through the year, reach a maximum around December-January, and then weaken. This is what happened through 2010.

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The first signs of a developing La Nina were evident by about May-June and by about September-October it was clear we were in a strong La Nina event. In fact, the 2010-11 La Nina was one of the strongest we have seen since records began in the late 19th century. This is what led the bureau to forecast an increased chance of a wet summer for 2010-11.

After the devastating eastern Australia floods of late 2010 and early 2011, the La Nina event started to weaken. The ''usual'' thing would have been for it to be followed by a more normal year, or even an El Nino with drier-than-average conditions. But it ''bounced'', and a new La Nina developed through the remainder of 2011. A similar ''bounce'' happened in 1973-75, also leading to back-to-back La Nina events, heavy rains and widespread floods.

So, perhaps global warming is off the hook? The floods of the past two years do result from natural features of the global climate system. But there have been changes to the climate system that are almost certainly due to increases in atmospheric carbon dioxide concentrations due to human activity.

One such change is the warming of the ocean around Australia. Ocean temperatures have increased about 0.7 degrees over the past 100 years. So the ocean temperatures around Australia over the past two years have been much warmer than was the case with La Nina events in the past (such as in 1973-75).

Climate scientists have reasons to suspect that warmer ocean temperatures can lead to increased rainfall, and the strong ocean warming we have seen around Australia has indeed been matched by a trend to more rainfall across the country. Annual rainfall, averaged across Australia, has gradually increased by about 25 per cent since the start of the 20th century (although not everywhere - the far south-west has dried through this period).

So it looks like global warming may be leading to a wetting trend across Australia, perhaps enhancing the heavy rains typically associated with La Nina events. But we are far from a scientific consensus on this.

It may be that the rainfall trend is just a fluke, even though it follows the warming trend closely. And even if global warming is enhancing Australia's rainfall on average, we should not conclude that droughts are behind us. Australia's natural climate variations are so strong that we will certainly see long and painful droughts in the future, as well as wet periods. There is also concern that global warming will lead to increased droughts in specific areas such as the deep south of the continent.

One way we can adapt to climate change (whether it brings drier or wetter conditions) is to make better use of the climate predictability we get from ENSO. By using the bureau's seasonal climate outlooks, we will be better adapted to current climate variations, so the impact of any exacerbation by global warming will be diminished.

Likewise, taking better advantage of the rapidly improving short-term weather forecasts from the bureau can help us prepare for floods better than we have been able to do in the past.

This again will help us adapt, irrespective of whether climate change brings worse floods.

The jury is still out on whether global warming is guilty of exacerbating the heavy rains associated with La Nina. But I wouldn't be letting it out on bail just yet. Instead, I hope we will take full advantage of what we know about La Nina and El Nino to offset any exacerbation that global warming might cause.

Professor Neville Nicholls is in the school of geography and environmental science at Monash University.